Now in XAML I can bind the ComboBox’ IsDropDownOpen-Property to the IsDropDownOpen-Property of the MainViewModel that is in the DataContext. In addition I bound a Checkbox to the IsDropDownOpen-Property of the MainViewModel to be able to change the value of that property:

That’s it. When I check the CheckBox, the Dropdown-List opens. Instead of changing the IsDropDownOpen-Property of the MainViewModel with a CheckBox, you could also set the IsDropDownOpen-Property in the MainViewModel when for example a Command is executed.